1. Field of the Invention
This invention relates to a ceramic lid for sealing a semiconductor device mount portion of a package substrate accommodating a semiconductor device and a semiconductor package using this lid.
2. Description of the Prior Art
A semiconductor device such as an integrated circuit is accommodated in a semiconductor device mount portion disposed on a package substrate, and is put to practical application after this mount portion is hermetically sealed by the ceramic lid (hereinafter called the "lid"). When heat-resistance, durability and reliability are required, ceramics such as alumina have been used as the material of the package substrate and the lid. When the ceramic substrate is sealed by the ceramic lid, a solder is used as a sealing material. Because the solder cannot be bonded directly to the ceramics, means for depositing an underlying metallized layer has been generally employed for the solder seal portions of the package substrate and the lid. Accordingly, the package substrate and the lid are bonded to each other by the solder through the underlying metallized layer.
The underlying metallized layer of the package substrate employs the structure comprising a metallized layer of a refractory metal such as W, Mo, etc, which can be cofired with the ceramic greensheets, and a Ni plating and an Au plating applied to the metallized layer. This structure secures bondability with the solder at the time of sealing and long-term reliability as a package. A metal such as Au, Ag--Pd, etc, is used in most cases as the underlying metallized layer of the lid.
The structure of the semiconductor package according to the present invention will be explained with reference to FIGS. 1 and 2. A semiconductor device 8 is stored in a semiconductor device mount portion 7 as a recess of a package substrate 2, and the semiconductor device mount portion is hermetically sealed by a lid 1. As shown in FIG. 1, the lid 1 includes an underlying metallized layer 5 disposed on peripheral edge portions (outer peripheral portion of an upper surface and side surfaces) 4 of a ceramic substrate 3, and a seal layer 6 as a sealing member disposed on the underlying metallized layer 5. A method of sealing the semiconductor device accommodated in the package substrate by the lid 1 is as follows. The semiconductor device 8 is first accommodated in, and mounted to, the semiconductor device mount portion 7 of the package substrate 2, and then a seal layer 6 of the lid is so superposed as to face an underlying metallized layer 10 formed on the upper surface of the package substrate 2. The lid 1 and the package substrate 2 are fixed by fixing members such as springs, clips, or the like. The solder is melted under such state through a heating furnace and is then cooled. In this way, the semiconductor device 8 is hermetically sealed inside the package substrate.
If the temperature is excessively high during this heating operation, the semiconductor device is likely to be damaged or broken. Accordingly, Japanese Patent Laid-Open No. 144966/1993 proposes a package characterized by using a solder having a low melting point. In other words, this reference disposes a package sealed by a solder alloy comprising, as a chemical composition, 1.0 to 10.0 wt % of at least one of Sn and In, 1.0 to 10.0 wt % of Ag, 3.0 to 13.0 wt % of Bi and the balance of Pb. The melting point of this solder is around 270.degree. C.; hence, its melting point is said to be lower by about 25.degree. C. than the melting points of conventional solders. Ag--Pd is concretely used as the underlying metallized layer of the lid.
Japanese Patent Laid-Open No. 145747/1991 discloses a lid which utilizes Ag--Pt as an underlying metal of an economical lid having high reliability, and which is equipped with a solder layer of a low melting-point solder chemically adaptive to the underlying metal, having high wethability with an Au plating as the underlying metal of the package substrate and comprising 4.0 to 6.2 wt % of Sn, 1.5 to 5.0 wt % of In, 1.5 to 3.0 wt % of Ag and the balance of Pb. Ag--Pt (Pt: 1 to 2 wt %) as the underlying metal has a characterizing feature in that its material cost is by far lower than Ag--Pd (Pd: 20 to 30 wt %).
Characteristics of a sealing solder layer on a lid and sealing characteristics between this lid and a semiconductor package substrate must satisfy the following requirements:
(1) High bondability can be obtained between the solder layer and the underlying metallized layer when the solder layer is formed on the underlying metallized layer by reflow, for the lid. PA1 (2) When the package substrate is sealed by the lid, good bondability should be obtained between the solder and the underlying metallized layer of the package substrate. PA1 (3) Sealing can be made at a temperature as low as 300.degree. C. or below. PA1 (4) Seal performance can be stably maintained for a long time for the sealed package.
The package disclosed in Japanese Patent Laid-Open No. 144966/1993 described above involves the following problems. In the case of the combination of an (Sn, In)--Ag--Bi--Pb system 4- or 5-component system solder with the Ag--Pd underlying metallized layer, Sn and In as the solder alloy components and Pd of the underlying metallized layer are likely to undergo alloying, and solder leaching is likely to occur and the underlying metallized layer is corroded. Accordingly, bondability drops, and stable seal performance cannot be obtained for a long term.
In the lid described in Japanese Patent Laid-Open No. 145747/1991, the melting point of the solder used is between 300.degree. and 350.degree. C. Therefore, the lid cannot satisfy the requirement for seal at a temperature lower than the temperature range described above.